This invention relates to dynamic random access memory devices, and, more particularly, to a method and system for allowing a memory device to be quickly and easily switched into and out of a low power, half density, operating mode.
As the use of electronic devices, such as personal computers, continue to increase, it is becoming ever more important to make such devices portable. The usefulness of portable electronic devices, such as notebook computers, is the limited by the limited length of time batteries are capable of powering the device before needing to be recharged. This problem has been addressed by attempts to increase battery life and attempts to reduce the rate at which such electronic devices consume power.
Various techniques have been used to reduce power consumption in electronic devices, the nature of which often depends upon the type of power consuming electronic circuits that are in the device. For example, electronic devices, such a notebook computers, typically include dynamic random access memory (xe2x80x9cDRAMxe2x80x9d) devices that consume a substantial amount of power. As the data storage capacity and operating speeds of DRAM devices continues to increase, the power consumed by such devices has continued to increase in a corresponding manner.
A variety of operations are performed in DRAM devices, each of which affects the rate at which the DRAM device consumes power. One operation that tends to consume power at a substantial rate is refresh of memory cells in the DRAM device. As is well-known in the art, DRAM memory cells, each of which essentially consists of a capacitor, must be periodically refreshed to retain data stored in the DRAM device. Refresh is typically performed by essentially reading data bits from the memory cells in each row of a memory cell array and then writing those same data bits back to the same cells in the row. This refresh is generally performed on a row-by-row basis at a rate needed to keep charge stored in the memory cells from leaking excessively between refreshes. Since refresh essentially involves reading data bits from and writing data bits to a large number of memory cells refresh tends to be a particularly power-hungry operation. Thus many attempts to reduce power consumption in DRAM devices have focused on reducing the rate at which power is consumed during refresh.
The amount of power consumed by refresh also depends on which of several refresh modes is active. A Self Refresh mode is normally active during periods when data are not being read from or written to the DRAM device. Since portable electronic devices are often inactive for substantial periods of time, the amount of power consumed during Self Refresh can be an important factor in determining how long the electronic device can be used between battery charges.
One technique that has been used to reduce the amount of power consumed by refreshing DRAM memory cells is to vary the refresh rate as a function of temperature. As is well known in the art, the rate at which charge leaks from a DRAM memory cell increases with temperature. The refresh rate must be sufficiently high to ensure that no data is lost at the highest temperature in the specified range of operating temperatures of the DRAM device. Yet, DRAM devices normally operate at a temperature that is substantially lower than their maximum operating temperature. Therefore, DRAM devices are generally refreshed at a rate that is higher than the rate actually needed to prevent data from being lost, and, as a result, unnecessarily consume power. To address this problem, some commercially available DRAM devices allow the user to program a mode register to select a lower maximum operating temperature. The DRAM device then adjusts the refresh rate to correspond to the maximum operating temperature selected by the user.
Although adjusting the refresh rate as a function of temperature does reduce the rate of power consumed by refresh, it nevertheless still allows power to be consumed at a significant rate for several reasons. For example, although the refresh rate may be reduced with reduced maximum operating temperature, the refresh may still result in refreshing a large number of memory cells that are not actually storing data.
Another approach to reducing the rate at which power is consumed by a refresh operation is to refresh less than all of the memory cells in the DRAM device in attempt to refresh only those memory cells needed to store data for a given application. As described in U.S. Pat. No. 5,148,546 to Blodgett, a software program being executed in a computer system containing the DRAM devices is analyzed to determine the data storage requirements for the program. The DRAM device then refreshed only those rows of memory cells that are needed to store data. In another approach, the DRAM device may be operated in a partial array self refresh (xe2x80x9cPASRxe2x80x9d) mode. In the PASR mode, a mode register is programmed by a user to specify a bank or portion thereof of memory cells that will be used and thus must be refreshed. The remaining memory cells are not used and thus need not be refreshed during at least some refresh modes. Although these techniques for refreshing less than all of the memory cells in a memory device can substantially reduce the rate of power consumption, it can nevertheless require a substantial amount of power to refresh the cells that are to be refreshed.
Still another technique that can be used to reduce the rate of refresh involves operating DRAM devices in a half density mode. A DRAM device that may be operated in a half density mode is described in U.S. Pat. No. 5,781,483 to Shore. In the half density mode, the low order bit of each row address, which normally designates whether the addressed row is even or odd, is ignored, and both the odd row and adjacent even row are addressed for each memory access. In a folded digit line architecture, activating an odd row will couple each memory cell in the row to a respective digit line, and activating an even row will couple each memory cell in the row to a respective complimentary digit line. Thus, for example, writing a xe2x80x9c1xe2x80x9d to an addressed row and column would result in writing a logic xe2x80x9c1xe2x80x9d voltage level to the memory cell in the addressed odd row and writing a logic xe2x80x9c0xe2x80x9d logic level to the memory cell in the addressed even row. Reading from the addressed row and column results in a logic xe2x80x9c1xe2x80x9d voltage level being applied to the digit line for the addressed column and a logic xe2x80x9c0xe2x80x9d voltage level being applied to the complimentary digit line for the addressed column. Therefore, in the half density mode, a sense amplifierlifier coupled to the digit line and complimentary digit line for each column receives twice the differential voltage that it normally receives when reading a memory cell at an addressed row and column.
The patent to Shore describes the use of the half density mode for the purpose of allowing the DRAM device to be used despite the presence of defective memory cells. If a memory cell in an addressed row and column is defective, the data bit stored in that memory cell can still be recovered from the other memory cell in the addressed row and column. However, it has been recognized that the half density mode can be used to reduce that rate at which power is consumed during refresh. Although a refresh in the half density mode requires twice as many memory cells to be refreshed for a given amount of stored data, the required refresh rate is less than half the required refresh rate when the DRAM device is operating in the full density mode. The substantially lower refresh rate required in the half density results from the increased differential voltage that is applied to the sense amplifierlifiers in the half density mode, as previously explained. As a result, the memory cells can be allowed to discharge to a greater degree between refreshes without the data bits stored therein being lost. Therefore, storing data in the half density mode can reduce the rate of power consumption during refresh
In conventional DRAM devices, the density mode, i.e., either half or full, is generally determined prior to sale of the device. If the power consumption of the DRAM device is of concern, the half density mode can be selected. Otherwise, the full density mode can be selected. Yet many power management algorithms for electronic devices containing DRAM devices, such as notebook computers, switch to a low power mode when the electronic device is inactive and back to a high power mode when the electronic device is active. It is therefore necessary for electronic devices to be able to frequently switch back and forth between low power and high power modes.
In conventional DRAM devices, it is not possible to switch between a full density mode and a half density mode. This limitation may be due to the difficulty in making this transition. The difficulty of being able to rapidly switch between the full density mode and the half density mode primarily results from two requirements. First is the need to first free-up alternate rows of memory cells into which data from an adjacent row of memory cells can be transferred for half density storage. The second requirement is the need to transfer data from the memory cells in a row storing data to a memory cell in the adjacent row once the adjacent row has been freed up by transferring data to another row. More particularly, if the DRAM device is operating in the full density mode, generally data will be stored in both even rows and odd rows of memory cells. To switch to the half density mode would require that the data stored in the even rows of memory cells, for example, be transferred to empty odd rows of memory cells. It would then be necessary to read the data stored in each odd row, and write the read data to corresponding memory cells in the adjacent even row. Transferring data between memory cells in this manner by conventional read/write operations would require a great deal of time and would therefore preclude quickly switching back and forth between the full density mode and the half density mode. Also, transferring approximately half of the data stored in the DRAM device by conventional read/write operations, which would be necessary to switch from the full density mode to the half density mode, would itself consume a great deal of power. While more efficient row copy schemes have been proposed for test purposes, such as the row copy scheme described in U.S. Pat. No. 5,381,368 to Morgan et al., these row copy schemes are generally suitable only when the same data or a repeating pattern of data are to be written to the entire array of memory cells. Yet switching from the full density mode to the half density mode would require transferring many rows of disparate data bits to respective adjacent rows after freeing up the adjacent rows by transferring the disparate data bits to other rows. It therefore does not seem possible to easily transition between the half density mode and the full density mode.
There is therefore a need for a power-saving technique that would allow switching into and out of a half density, low refresh rate mode without requiring time and power consuming reading and writing of data to a second set of memory cells.
A system and method according to the invention allows a DRAM device to be easily and quickly switched back and forth between a full density mode consuming power at a relatively fast rate and a half density mode consuming power at a relatively slow rate. The row addresses applied to the DRAM device are reordered by remapping the most significant bit of each row address to the least significant bit of the row address during all operating modes. As a result, all of the odd (or even) rows of the DRAM array are populated with data before any of the even (or odd) rows are populated with data. As long as the data stored in the DRAM device uses less than half of the capacity of the DRAM device, data will then be stored only in alternate rows, and the row adjacent each row in which data are stored will be free to store data. When the DRAM device is to be switched from the full density mode to the half density mode, data stored in each row is simply transferred to the adjacent row. Thereafter when operating in the half density mode, the row corresponding to each row address and the adjacent row are accessed at the same time. Although the data stored in each row can be transferred to the adjacent row by a variety of techniques, it is preferably transferred by transferring the data from each row to the adjacent row during the first refresh of the row. More particularly, when a row is first refreshed after the DRAM device has been switched to the half density mode, the sense amplifierlifiers are left active so that the voltage levels corresponding to the data stored in the memory cells being refreshed are maintained on the respective digit line pairs. The adjacent row is then activated thereby transferring the voltage on the digit lines to the memory cells in the adjacent row. Once the data have been transferred to the adjacent rows during refresh at the full density refresh rate, the refresh rate can be significantly reduced during operation in the half density mode.